Bandstop filter

ABSTRACT

A bandstop filter where variation in characteristics is suppressed to minimum and which realizes an increased production yield. The physical length of a line joint portion between a main line and an oscillator can be enlarged by providing an impedance non-continuous structure portion in a strip conductor of the oscillator. In comparison to the case where the impedance non-continuous structure portion is not provided, the width of a joint slit required to obtain an equal joint amount can be enlarged. When the joint slit width is enlarged, variation in filter characteristics caused by pattern accuracy can be reduced because of the enlarged joint slip width, thus improving a filter yield. This means that pattern accuracy requirement for production is loosened. Freedom in selecting a dielectric substrate is increased, which also provides an advantage that a filter can be produced using a less expensive dielectric substrate with not very high pattern accuracy.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a high-frequency filter used in amicrowave band and a millimeter-wave band.

2. Description of the Related Art

With a bandstop filter described in a document entitled “Exact Design ofBand-stop Microwave Filters” (written by B. M. Schiffman and G L.Matthaei in IEEE Trans. on MTT, vol. MTT-12, pp 6-15 (1964)), forinstance, by reflecting a signal in a frequency band in which theelectrical length of an inner conductor of a resonator becomesapproximately 90 degrees, passage of the signal in the frequency band isinhibited.

In the case of this bandstop filter, a frequency, at which the resonatorresonates, becomes the center frequency of a stop band. Also, a gap of aportion, in which the inner conductor of the resonator and an innerconductor of a main line are arranged parallel to each other andconstitute a line joint, corresponds to the stop bandwidth of thefilter. That is, there is a property with which it is possible toenlarge the stop bandwidth by enlarging the joint between the resonatorand the main line through reduction of the gap of the line jointportion.

Further, the joint between the resonator and the main line describedabove becomes the maximum when the electrical length in the line jointportion at the center frequency of the stop band is 90 degrees. That is,when it is desired to secure a predetermined joint amount between themain line and the resonator in the case where the electrical length inthe line joint portion at the center frequency of the stop band issmaller than 90 degrees, it is required to reduce the gap of the linejoint portion likewise.

However, the conventional technique has the following problems. The sizeof the gap of the line joint portion described above depends on the kindof the line constituting the filter. In addition, because of theproducable minimum size, production errors, and the like, it is notguaranteed that the size of the gap necessarily becomes a desired size.This imposes a limitation on the stop bandwidth that is realizable witha produced filter.

In particular, when the conventional bandstop filter is constructedusing a planar circuit such as a microstrip line or a strip line, therearise the following problems. That is, a strip conductor correspondingto the inner conductor described above has an extremely thin thickness,which makes it more difficult to obtain a large joint. When a gap forrealizing a desired stop bandwidth is reduced and approaches alimitation in terms of production, a problem of variation in gap due toa production error or variation in width due to a production error oftwo strip conductors becomes more prominent. As a result, variation incharacteristics due to the variation leads to variation in stop bandfrequency. However, it is difficult to adjust the distance between thestrip conductors after formation because they are formed through etchingor the like. Therefore, the variation in characteristics due to theproduction error directly leads to a filter yield reduction.

In addition, the conventional bandstop filter has a problem in that aproduction error in short-circuiting means of the resonator directlyleads to variation in filter characteristics. In particular, when thefilter is constructed using a planar circuit such as a microstrip line,the short-circuiting means is formed using a through hole or a via hole.In such a case, there is a problem in that when the positional relationbetween the strip conductor and the through hole (via hole) changes dueto a problem in terms of production, a resonance frequency is shiftedand there occurs characteristic deterioration such as variation in stopband.

SUMMARY OF THE INVENTION

The present invention has been made to solve the above problems, and hasan object to provide a bandstop filter with which variation incharacteristics is suppressed to minimum and a production yield isimproved.

A bandstop filter according to the present invention includes: a mainline connecting an input terminal and an output terminal to each other;and a ¼ wavelength resonator arranged in proximity to the main lineapproximately parallel to the main line with a distance of anapproximately ¼ wavelength, in which the ¼ wavelength resonator includesa first impedance non-continuous structure portion and divides a linesection that is approximately parallel to the main line into portionshaving different characteristic impedances.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an internal construction diagram of a bandstop filteraccording to a first embodiment of the present invention;

FIG. 2 is an enlarged view of a resonator in the second stage of thebandstop filter according to the first embodiment of the presentinvention;

FIG. 3 is an equivalent circuit diagram of the bandstop filter accordingto the first embodiment of the present invention;

FIG. 4 is a circuit diagram for explanation of design of a resonatorportion of the bandstop filter according to the first embodiment of thepresent invention;

FIG. 5 shows the reflection characteristic and transmissioncharacteristic of the bandstop filter according to the first embodimentof the present invention;

FIG. 6 is an internal construction diagram of a bandstop filteraccording to a second embodiment of the present invention;

FIG. 7 is an equivalent circuit diagram of the bandstop filter accordingto the second embodiment of the present invention;

FIG. 8 is an internal construction diagram of a bandstop filteraccording to a third embodiment of the present invention;

FIG. 9 is an equivalent circuit diagram of the bandstop filter accordingto the third embodiment of the present invention;

FIG. 10 is an internal construction diagram of a bandstop filteraccording to a fourth embodiment of the present invention;

FIG. 11 is an enlarged view of a resonator in the second stage of thebandstop filter according to the fourth embodiment of the presentinvention;

FIG. 12 is an internal construction diagram of a bandstop filteraccording to a fifth embodiment of the present invention; and

FIG. 13 is an enlarged view of a resonator in the second stage of thebandstop filter according to the fifth embodiment of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described withreference to the drawings.

First Embodiment

FIG. 1 is an internal construction diagram of a bandstop filteraccording to the first embodiment of the present invention, with a viewfrom above and a cross-sectional view being illustrated. In FIG. 1, abandstop filter including three resonators is illustrated. Eachconstruction element of the first resonator is given a reference numeralwith a suffix “a” and suffixes “b” and “c” are used for the second andthird resonators in a like manner. Note that in the followingdescription, when an explanation that is common to the three resonatorsis made, only reference numerals, from which the suffixes are removed,are used.

The bandstop filter of the first embodiment is a three-stage filterhaving a microstrip line structure constructed using one dielectricsubstrate 9. An input signal to be bandstopped is taken into thebandstop filter from an input terminal 5 _(IN), passes through a stripconductor 1 of a main line, and is finally outputted as a bandstoppedsignal from an output terminal 5 _(OUT). There are strip conductors 2 ato 2 c of resonators in three stages arranged approximately parallel tothe strip conductor 1 of the main line and bandstopping to be describedin detail later is performed through operations thereof.

The bandstop filter of the first embodiment is constructed using amicrostrip line structure including an earth conductor 6 on one mainsurface of the dielectric substrate 9 and including the strip conductor1 of the main line and the strip conductors 2 a to 2 c of the resonatorson the other main surface. The strip conductors 2 a to 2 c of theresonators are short-circuited with the earth conductor 6 byshort-circuiting means 3 a to 3 c through through holes 8 a to 8 c,respectively.

FIG. 2 is an enlarged view of the resonator in the second stage of thebandstop filter according to the first embodiment of the presentinvention. The short-circuiting means 3 b for short-circuiting betweenthe strip conductor 2 b of the resonator and the earth conductor 6 isarranged at one end of the strip conductor 2 b of the resonator. On theother hand, the other end of the strip conductor 2 b of the resonator isset as an open end 4 b. Also, the strip conductor 1 of the main line andthe strip conductor 2 b of the resonator are placed under a positionalrelation in which they are approximately parallel to each other with adistance corresponding to a gap of a joint slit 7 b that is a gapbetween the strip conductor 1 and the strip conductor 2 b. In FIG. 2,the gap of the joint slit 7 b is expressed as “S1”.

Further, the strip conductor 2 b of the resonator has an impedancenon-continuous structure portion 10 b. By reducing the width of thestrip conductor 2 b of the resonator in a section from the impedancenon-continuous structure portion 10 b to the open end 4 b, the impedancein this section is increased:

FIG. 3 is an equivalent circuit diagram of the bandstop filter accordingto the first embodiment of the present invention. The even modeimpedance, odd mode impedance, and electrical length of the line jointof each resonator are expressed as “Ze”, “Zo”, and “θ”, respectively.

Also, FIG. 4 is a circuit diagram for explanation of design of theresonator portion of the bandstop filter according to the firstembodiment of the present invention, with the illustrated circuitdiagram corresponding to one resonator. Further, FIG. 5 shows thereflection characteristic and transmission characteristic of thebandstop filter according to the first embodiment of the presentinvention.

Next, an operation of the bandstop filter will be briefly described withreference to these drawings. At the first resonator in FIG. 1, amonghigh-frequency signals inputted from the input terminal 5 _(IN), asignal at a frequency at which the electrical length of the stripconductor 2 a of the resonator becomes sufficiently smaller than 90degrees, that is, a frequency, at which the electrical length of thestrip conductor 2 a of the resonator becomes sufficiently smaller than a¼ wavelength, is transferred to the resonator in the next stage (or anoutput terminal 5 _(OUT) side) almost as it is. In the case of theequivalent circuit diagram in FIG. 3, a frequency band, in which theelectrical length θ1 becomes sufficiently smaller than 90 degrees,corresponds to this. This phenomenon is due to the following reason.Because of the existence of the resonator, a shunt capacity is added tothe main line. Also, a portion of the strip conductor 1 of the main linethat faces the strip conductor 2 a of the resonator with a joint slit 7a in-between is adjusted so that it assumes an impedance that isslightly higher than the design impedance (terminal condition) of thefilter. Consequently, a slight series inductance is exhibited, sothrough combination of the shunt capacity and the series inductance,impedance matching analogous to the frequency band of the pass band of alow pass filter is performed.

Also, among the high-frequency signals inputted from the input terminal5 _(IN), a signal at a frequency at which the electrical length of thestrip conductor 2 a of the resonator becomes approximately 90 degrees,that is, a frequency, at which the electrical length of the stripconductor 2 a of the resonator becomes approximately a ¼ wavelength, istrapped in the resonator because the resonator resonates. Then, almostall of energy of the signal other than a part of the energy dissipateddue to a loss in the resonator is reflected toward the input terminal 5_(IN). For a circuit, the shunt capacity added to the main line throughthe existence of the resonator becomes extremely large and a state isobtained in which the main line is short-circuited or is nearlyshort-circuited in a portion on a short-circuiting means 3 a side of thejoint slit 7 a in which the strip conductor 1 of the main line and thestrip conductor 2 a of the resonator face each other in parallel.Consequently, almost all of the energy is reflected (see FIG. 5).

Further, among the high-frequency signals inputted from the inputterminal 5 _(IN), a signal at a frequency at which the electrical lengthof the strip conductor 2 a of the resonator becomes sufficiently largerthan 90 degrees, that is, a frequency, at which the electrical length ofthe strip conductor 2 a of the resonator becomes sufficiently largerthan a ¼ wavelength, is transferred to the resonator in the next stage(or the output terminal 5 _(OUT) side) almost as it is. In the case ofthe equivalent circuit diagram in FIG. 3, a frequency band, in which theelectrical length θ1 becomes sufficiently larger than 90 degrees,corresponds to this. This phenomenon is due to the following reason. Theresonator is arranged parallel to the main line and the electricallength of the resonator is larger than 90 degrees, so a state isobtained in which a shunt inductance is added to the main line. Inaddition, a portion of the strip conductor 1 of the main line that facesthe strip conductor 2 a of the resonator with the joint slit 7 ain-between is adjusted so that it has an electrical length, which islarger than 90 degrees, and assumes an impedance that is slightly higherthan the design impedance (terminal condition) of the filter.Consequently, an electrical condition that is analogous to a seriesarrangement of capacitances is obtained and through combination of theshunt inductance and the series capacitance, impedance matchinganalogous to the frequency band of the pass band of a high pass filteris performed. Therefore, most of the energy of the inputted signal istransferred to the resonator in the next stage (or the output terminal 5_(OUT) side).

In addition, the bandstop filter according to the first embodiment ofthe present invention is characterized in that the resonator is providedwith the impedance non-continuous structure portion 10. With thischaracteristic construction, it becomes possible to enlarge the physicallength of the resonator and also enlarge the joint slit 7 as comparedwith a case where the resonator does not include the impedancenon-continuous structure portion 10.

Next, how the physical dimensions of the resonator and the physicaldimensions of the joint portion structure between the main line and theresonator of the bandstop filter in the first embodiment of the presentinvention are designed will be described.

In FIG. 4, an equivalent circuit when the resonator includes theimpedance non-continuous structure portion 10 is illustrated on the leftside and an equivalent circuit when the resonator does not include theimpedance non-continuous structure portion 10 is illustrated on theright side. In design of the resonator in the bandstop filter includingthe main line portion, dimensional parameters are selected so that theequivalent circuit when the resonator including the impedancenon-continuous structure portion 10 is used and the equivalent circuitwhen the resonator not including the impedance non-continuous structureportion 10 is used become electrically equivalent to each other at thecenter frequency of the stop band. In FIG. 4, the strip conductor widthis expressed as “W”, the joint slit width is expressed as “S”, thephysical length is expressed as “L”, the line joint even mode impedanceis expressed as “Ze”, the odd mode impedance is expressed as “Zo”, andthe electrical length is expressed as “θ”. Also, in the circuit diagramon the left side of FIG. 4, a suffix “s” of reference symbols indicatesa circuit corresponding to a short-circuiting means 3 b side withreference to the impedance non-continuous structure portion 10 b in FIG.2 and a suffix “o” of the reference symbols indicates a circuitcorresponding to an open end 4 b side with reference to the impedancenon-continuous structure portion 10 b in FIG. 2. Further, the circuitillustrated on the right side of FIG. 4 is a circuit uniquely giventhrough designation of the filter bandwidth, the number of stages, thereflection loss in the pass band, and the like based on a certainprocedure described in the document described above or the like.

The resonator including the impedance non-continuous structure portion10 is referred to as the “stepped impedance resonator” and is often usedas means for miniaturization of the resonator or the like. In the firstembodiment, in a ¼ wavelength resonator whose one end is short-circuitedand other end is opened, the impedance of the line on the open end 4side is set higher than the impedance of the line on theshort-circuiting means 3 side by the impedance non-continuous structureportion 10. Therefore, it becomes possible to enlarge the physicallength of the resonator with respect to a resonance frequency from thephysical length thereof with respect to the resonance frequency in acase where the impedance non-continuous structure portion 10 is notincluded. That is, by providing the impedance non-continuous structureportion 10, it becomes possible to enlarge the physical length of theline joint portion constructed between the main line and the resonator.

The joint amount of the line joint constructed between the main line andthe resonator fundamentally has a relation in which it is proportionalto the physical length of the line joint portion and is inverselyproportional to the width of the joint slit 7. Accordingly, when adesired joint amount between the main line and the resonator is secured,it is possible to enlarge the width of the joint slit 7 by enlarging thephysical length of the line joint portion through provision of theimpedance non-continuous structure portion 10. That is, the parametersof the physical dimensions in FIG. 4 have relations “(Ls+Lo)>L,Ss=So>S”.

As described above, by providing the impedance non-continuous structureportion 10 for the strip conductor 2 of the resonator, it becomespossible to enlarge the physical length of the line joint portionbetween the main line and the resonator. As a result, it becomespossible to enlarge the width of the joint slit 7 (corresponding to S1in FIG. 2) for obtainment of an equal joint amount as compared with acase where the impedance non-continuous structure portion 10 is notprovided. Consequently, with the bandstop filter of the firstembodiment, an effect is provided that it is possible to realize afilter with a large stop bandwidth, which requires an enlarged jointamount, under a state where the width of the joint slit 7 is enlarged ascompared with a conventional case. In addition, the enlargement of thewidth of the joint slit 7 makes it possible to reduce variation infilter characteristics caused by pattern accuracy, which provides aneffect that a filter production yield is improved. This corresponds tolooseness of a pattern accuracy requirement for production andflexibility in selection of a dielectric substrate is increased, whichbrings about an advantage that it is possible to produce a filter usingan inexpensive dielectric substrate with not very high pattern accuracy.

Second Embodiment

FIG. 6 is an internal construction diagram of a bandstop filteraccording to a second embodiment of the present invention, with a viewfrom above and a cross-sectional view being illustrated. Also, FIG. 7 isan equivalent circuit diagram of the bandstop filter according to thesecond embodiment of the present invention. The fundamental structure isthe same as that of the bandstop filter in the first embodiment. Thesecond embodiment differs from the bandstop filter in the firstembodiment in the following two points. That is, the number of stages ofthe filter is reduced to one and a tip-end open transmission line 11having an approximately ¼ wavelength is used in place of theshort-circuiting means.

The bandstop filter of the second embodiment performs fundamentally thesame operation as in the first embodiment. The tip-end open transmissionline 11 having the approximately ¼ wavelength is used in place of theshort-circuiting means and is placed under an open state by an open end14. In this state, the wavelength of the resonator at the centerfrequency of the stop band changes from the ¼ wavelength to a ½wavelength. In addition, the through hole for constructing theshort-circuiting means becomes unnecessary, production becomes easy, andthere occurs no variation in characteristics due to a production errorconcerning the short-circuiting means 3, such as an error of thediameter of the through hole 8 or an error of the positional relationbetween the through hole 8 and the strip conductor 2 of the resonator,in theory.

When the resonator is changed from the ¼ wavelength to the ½ wavelength,the joint amount that is required between the main line and theresonator is increased as compared with the case where the ¼ wavelengthresonator is used. This is because the frequency characteristics of thereactance of the resonator become steep. Therefore, it becomes necessaryto reduce the width of the joint slit 7 in accordance with the jointamount, which leads to a case where production becomes difficult due toa production limitation as to the minimum conductor distance. In otherwords, it is difficult to realize a filter having an enlarged stopbandwidth through reduction of the width of the joint slit 7. In thebandstop filter of the second embodiment, the physical length of theline joint portion is enlarged by providing an impedance non-continuousstructure portion 10 for the line joint portion, which makes it possibleto make up for a shortage of the joint amount. As a result, it becomespossible to enlarge the width of the joint slit 7.

With the structure of the bandstop filer of the second embodiment, theshort-circuiting means using a through hole or the like becomesunnecessary, which prevents variation in characteristics due to aproduction error as to the short-circuiting means and facilitatesproduction. In addition, as compared with the ¼ wavelength resonator,the ½ wavelength resonator requires a large joint amount between themain line and the resonator. In the present invention, however, theimpedance non-continuous structure portion is provided for the linejoint portion, which makes it possible to enlarge the joint amountwithout narrowing the joint slit. As a result, an effect is providedthat it is possible to realize a bandstop filter using a ½ wavelengthresonator with ease. In addition, the necessity to narrow the joint slitthan necessary is eliminated, which improves the production yield.

Third Embodiment

FIG. 8 is an internal construction diagram of a bandstop filteraccording to a third embodiment of the present invention, with a viewfrom above and a cross-sectional view being illustrated. Also, FIG. 9 isan equivalent circuit diagram of the bandstop filter according to thethird embodiment of the present invention. The fundamental structure isthe same as that of the bandstop filter in the second embodiment. Thethird embodiment differs from the bandstop filter in the secondembodiment in that an impedance non-continuous structure portion 13 isprovided for the tip-end open transmission line 11 in the secondembodiment.

The bandstop filter of the third embodiment performs fundamentally thesame operation as in the second embodiment and provides fundamentallythe same effect as in the second embodiment. In the bandstop filter ofthe third embodiment, the second impedance non-continuous structureportion 13 is provided for the tip-end open transmission line 11 that isa part of a ½ wavelength resonator. The impedance Zs2 of the tip endportion of the tip-end open transmission line 11 is set lower than theimpedance Zs1 of the portion on a main line side of the tip-end opentransmission line 11. With this construction including the secondimpedance non-continuous structure portion 13, the overall electricallength of the tip-end open transmission line 11 is reduced, whichprovides an effect that it is possible to obtain a compact filter.

Fourth Embodiment

FIG. 10 is an internal construction diagram of a bandstop filteraccording to a fourth embodiment of the present invention, with a viewfrom above and a cross-sectional view being illustrated. Also, FIG. 11is an enlarged view of a resonator in the second stage of the bandstopfilter according to the fourth embodiment of the present invention. Thefundamental structure is analogous to that of the bandstop filter in thefirst embodiment, but there are the following two points of difference.That is, in the fourth embodiment, the impedance non-continuousstructure portion 10 is not provided and the structure of theshort-circuiting means 3 is changed. In the bandstop filter of thefourth embodiment shown in FIG. 11, two short stubs 12 b-1 and 12 b-2constructed using through holes 8 b-1 and 8 b-2 and having shortelectrical lengths are arranged to oppose each other and are connectedto each other. In addition, the two short stubs 12 b-1 and 12 b-2 areconnected to a line joint portion between the main line and theresonator through a short transmission line.

With such a structure, as will be described below, an effect is providedthat even when the positional relation of the two through holes to theconductor pattern varies due to a production error, variation inresonator resonance frequency is suppressed to minimum and variation infilter characteristics is reduced. The reason why the variation inresonance frequency is small even when the positions of the throughholes with respect to the conductor pattern change is that thecharacteristics of the short-circuiting means are determined by the sumof the characteristics of the two short stubs 12 b-1 and 12 b-2. Forinstance, when the positions of the through holes are displaced in thehorizontal direction in FIG. 11, one short stub 12 b-1 (or 12 b-2) iselongated but the other short stub 12 b-2 (or 12 b-1) is shortened,which results in a situation where characteristic variations cancel outeach other. Also, when the positions of the through holes are displacedin the vertical direction in FIG. 11, this is a displacement in adirection orthogonal to the length direction of the short stubs 12 b-1and 12 b-2, so no significant change occurs to the electrical lengths ofthe short stubs 12 b-1 and 12 b-2. Therefore, even when the positions ofthe through holes 8 are displaced, the variation in characteristics issuppressed, which improves the production yield.

Fifth Embodiment

FIG. 12 is an internal construction diagram of a bandstop filteraccording to a fifth embodiment of the present invention, with a viewfrom above and a cross-sectional view being illustrated. Also, FIG. 13is an enlarged view of a resonator in the second stage of the bandstopfilter according to the fifth embodiment of the present invention. Thebandstop filter of the fifth embodiment has a fundamental structure inwhich the impedance non-continuous structure portion 10 used in thebandstop filter in the first embodiment is applied to the bandstopfilter in the fourth embodiment.

The bandstop filter of the fifth embodiment provides the same effect asthe bandstop filter in the first embodiment. In addition, like in thecase of the bandstop filter in the fourth embodiment, the bandstopfilter of the fifth embodiment provides an effect that variation incharacteristics ascribable to positional displacements of the throughholes with respect to the conductor pattern is reduced. When the shortstubs 12 b-1 and 12 b-2 are used as the short-circuiting means 3 like inthe fourth embodiment, the structure of the short-circuiting means 3increases in size, so it becomes inevitable to arrange theshort-circuiting means 3 at a position spaced apart from the stripconductor 1 of the main line due to a restriction under a productionrule. Consequently, the inductance of the short-circuiting means 3 isincreased, so it becomes necessary to shorten the physical length of theline joint portion that establishes a joint between the main line andthe resonator. When the physical length of the line joint portion isshortened, the joint slit 7 becomes small and the stop bandwidth of thefilter is limited. Therefore, when the short stubs 12 b-1 and 12 b-2described in the fifth embodiment and the fourth embodiment are used asthe short-circuiting means 3, the effect of making up for a shortage ofthe joint amount with the impedance non-continuous structure portion 10is increased. When it is assumed that the same stop bandwidth isrealized, the dimensions S4 and S5 of the slit joint portion 7 shown inFIGS. 11 and 13 greatly differ from each other. That is, it is possibleto set S5 larger than S4, which results in a possibility of producing abandstop filter having less variation in characteristics with ease. As aresult, the production yield is improved.

It should be noted here that in the above embodiments, a filter having amicrostrip line structure has been described, but it is of coursepossible to provide the same effect even when the filter is constructedusing another line structure such as a strip line or a coplanar line.

INDUSTRIAL APPLICABILITY

As described above, according to the present invention, it becomespossible to obtain a bandstop filter with which variation incharacteristics is suppressed to minimum and a production yield isimproved.

1. A bandstop filter comprising: a main line connecting an inputterminal and an output terminal to each other; and a ¼ wavelengthresonator arranged in proximity to the main line approximately parallelto the main line with a distance of an approximately ¼ wavelength,wherein the ¼ wavelength resonator includes a first impedancenon-continuous structure portion that divides a line section that isapproximately parallel to the main line into portions having differentcharacteristic impedances.
 2. The bandstop filter according to claim 1,wherein the ¼ wavelength resonator has a construction, in whichshort-circuiting means for short-circuiting with an earth conductor isprovided at one end and an open end is provided at the other end, and inthe line section that is approximately parallel to the main line, acharacteristic impedance in a line section on the open end side is sethigher than a characteristic impedance in a line section on theshort-circuiting means side.
 3. The bandstop filter according to claim1, wherein the ¼ wavelength resonator has a construction, in which atip-end open approximately ¼ wavelength line is provided at one end andan open end is provided at the other end, and in the line section thatis approximately parallel to the main line, a characteristic impedancein a line section on the open end side is set higher than acharacteristic impedance in a line section on the tip-end openapproximately ¼ wavelength line side.
 4. The bandstop filter accordingto claim 3, wherein the tip-end open approximately ¼ wavelength lineincludes a second impedance non-continuous structure portion and in aline section of the tip-end open approximately ¼ wavelength line, acharacteristic impedance in a line section on an open end side of thetip-end open approximately ¼ wavelength line is set lower than acharacteristic impedance in a line section on the main line sidethereof.
 5. The bandstop filter constructed using aplanar-circuit-shaped line including a dielectric substrate, stripconductors, and at least one earth conductor, comprising: a stripconductor of a main line connecting an input terminal and an outputterminal to each other; and a strip conductor of a ¼ wavelengthresonator arranged in proximity to the main line approximately parallelto the main line with a distance of an approximately ¼ wavelength,wherein the strip conductor of the ¼ wavelength resonator has aconstruction in which short-circuiting means for short-circuiting withthe earth conductor is provided at one end and an open end is providedat the other end; and the short-circuiting means includes two shortstubs which each have a through-hole that electrically connects thestrip conductor of the ¼ wavelength resonator and the earth conductor toeach other.
 6. The bandstop filter according to claim 5, wherein thestrip conductor of the ¼ wavelength resonator includes a first impedancenon-continuous structure portion that divides a line section that isapproximately parallel to the strip conductor of the main line intoportions having different characteristic impedances.